class animal(object):
    def run(self):
        print 'animal is run'
class Dog(animal):
    def run(self):
        print 'Dog is running'
class Husky(Dog):
    def run(self):
        print 'Husky is running'
a=animal()
d=Dog()
h=Husky()
#a.run()
#d.run()
#h.run()
isinstance(h,Dog)
len('xxx')
'xxx'.__len__()